1. Field of the Invention
This invention relates to a sensor for measurement of magnetic fields, and in particular to a magnetic field sensor using a magnetoresistive (MR) element and associated flux collectors.
2. Description Relative to the Prior Art
A wide variety of sensors are known in the art for use in measuring magnetic fields. Among these are the Hall effect sensor, proton resonance effect sensor, superconducting quantum interference detector (SQUID), fluxgate magnetometer, inductive pickup, magnetoresistive sensor and others. Depending upon the application, one or another of the above sensors may be chosen for a particular magnetic field measurement.
Among these sensors, the MR sensor has proven to be one of the most versatile magnetic field detectors. The single strip MR conventional element is relatively simple to fabricate, has a flat frequency response to very high frequencies, and has acceptable sensitivity, i.e. signal voltage per unit signal field, for typical field measurements. For low amplitude magnetic field measurements, however, the sensitivity of an MR sensor of such a single strip configuration is limited. Large area MR sensors have been developed where the MR element is deposited on a substrate in a meandering pattern, increasing the length of the MR element many fold compared to a conventional single strip MR. This configuration increases the total resistance of the sensor, and as the signal is proportional to the change of resistance with applied magnetic field, the sensitivity of the sensor is correspondingly improved. However, as the signal-to-noise ratio is proportional to the square root of power, power dissipation increases more rapidly than does the sensitivity with increasing resistance. Additionally, the large area sensor takes up a relatively large volume due to the increased surface occupied by the MR element, precluding the possibility of its use as a small probe for measuring the magnetic field in a localized space. Such a large area sensor is described by Kwiatkowski and Tumanski, "Journal of Physics E: Scientific Instruments", Great Britain, Volume 19, 1986, page 510.
It is known in the prior art that the sensitivity of an MR sensor may be improved by providing high permeability flux collectors in line with the MR element. The prior art teaches the use of bulk permalloy rod flux collectors with dual MR sensors such that the overall length of the sensor assembly is approximately 20 centimeters. (O. Gebhardt, W. Richter, "Experimentelle Technik der Physik", volume 30, 1982, pages 363-367.) It will be appreciated that as with the large area sensor, the dual MR sensor with bulk permalloy rod flux collectors, being 20 centimeters in length, is unsuitable as a probe for field measurement in restricted volumes.